Sound-reproducing apparatus



April 24. 1928.

a. E. snown SOUND REPRODUCING APPARATUS Filed Oct. 1. 1926 3 Sheets-Sheet 1 'INVEN OR 6. BY a Toma v 3 Sheets-Sheet 2 B. E. BROWN Filed Oct. 1. 1926 SOUND REPRODUCING APPARATUS April 24, 1928.

l VEN OR ATTORNEY 5 Sheets-Sheet: s

B. E. BROWN Filed Oct. 1. 1926 April 24, 1928.

SOUND REPRODUCING APPARATUS transmitted .present invention has to do with the last Patented Apr; 24, 1928.

1,667,531. UNITED STATES PATENT OFFICE.

BEN E. BROWN, OF MOUNTAIN LAKES, 'NEW JERSEY, ASSIGNOB TO B. B. LABORA- TORIES,'INC., OF EAST ORANGE, NEW- JERSEY, A CORPORATION OF NEW JERSEY.

SOUND-REPRODUCING APPARATUS.

Application filed October 1, 1926. Serial No. 138,882.

The present invention relates to sound reproducing apparatus for use in radio receiving sets,\phonographs, and the like, and has to do with the conversion of energy from one form to another and particularly with the conversion into sound of the impulses. of electrical energy which occur in the apparatus and which represent the sounds being reproduced.

Radio receiving sets, for example, are designed to receive or detect incoming mpulses of electrical energy representing sounds, which impulses proceed from the transmitting station, and'to pass such impulses into 'a device capable of translating or converting themback into sounds which are intended to duplicate and thus reproduce the original sounds generated for trans-. mission. The relatively feeble incoming impulses are commonly amplified 1n the radio receiving set by means of vacuum tubes or the like before being passed to the part of the appar'atuswhich converts them into sound to thus facilitate the generation of sound in sufficient volume to be conveniently heard and enjoyed by the listener. The device for converting the impulses of electrical energy into sound may assume various forms, as, for instance, ordinary telephone receivers introduced into the elec missionof such impulses as well as upon the' accurate reception and conversion of the lmpulses into sounds. The

mentioned step in this process of modifying and converting energy from one form to another, namely, the conversion of the im;

pulses of. electrical energy in the receiving apparatus into sound.

It is an important object'of the present invention to provide a sound reproducing device or unit of the class just referred to which converts the impulses of electrical en-- ergy occurring in the receiving apparatus a into sound more accurately than is possible in apparatus of this general class as hereto:

fore proposed, to thus re roduce the original sounds more faithfu ly and naturallyin all of their qualities. Distortion or poor quality in the reproduced sounds, which originates in the sound producing unit, arises principally from two causes. The first of these causes is the failure of the apparatus to handle and reproduce sounds of diflerent pitch in an identical manner. If, for instance, an apparatus freely handles and satisfactorily reproduces the higher frequencies, thatis, sounds of relatively high pitch, but does not freely handle the lowerfrequencies but rather offers a resistance to their passage through the apparatus and thus reproduces the lower sounds to a lesser extent than it does thesounds of higher pitch, the result is that the reproduction of the original sounds is not accurate. The deficiency in the low 'tones produces an unbalanced and an unnatural effect in the reproduced sound which greatly impairs thev quality-of the production. The second of these causes is the introduction 'into the reproduced sounds of additional frequencies or harmonics or of additional quantities of frequencies or harmonics already present.- Harmonics aresounds of" frequencies which are multiples of the frequencies occurring in the ori inal sounds intended to be introduced. T e effect of the introduction .of these harmonics is principally a change in the qualityof the sounds which renders the reproduced sounds to a certain extent unlike the original.

into sound in\ such manner that all frequencles are converted and reproduced more nearly to the same extent and with a lesser.

introduction of undesirable harmonics than has been heretofore accomplished. .If the electrical impulses in the receiving apparatus fairly represent the original sounds, the present apparatus continues the reproduction satisfactorily and produces as its ultimate result sounds which in a fair sense duplicate the original sounds intended to be reproduced.

It is a further object of the presentinven- 'tion to provide a sound reproducing apparatus of the class described which is of maximum efficiency; that is, anapparatus which will produce-a maximum sound out- ..put for a given power input as determined y the electrical energy supplied to it from The ap aratus of the present invention converts t e electrical impulses the receiving apparatus. practical result of this is that a satisfactorily loud reproduction may be obtained with the use of a lower electric current in the output circuit of the vacuum tubes of the radio set.

Anexcessive output current often results in over-loading of the vacuum tubes, par:

and compact construction, is capable of handling a large amount of energy and is designed throughout in every detail of mechanical and electrical structure to attain the results above noted. The invention, therefore, contemplates all of the various novel features of electrical and structural design which function to accomplish the improved reproduction which is effected by the apparatus.

.A preferred embodiment of the invention is'shown in the accompanying drawings in which Figure 1 is a side view of the apparatus with the main portion ofthe housing removed; Figures 2 and 3 are end views taken from the left and right ends, respectively, of Figure 1; Figure 4 is a view generally similar toFigure 1 takenfrom the opposite side of the apparatus; Figure 5 is a transverse sectional view taken on line 5-5 of Figure 4; Figure 6 is a longitudinal sectional view taken on line 6-6 of Figure 3; Figures 7 and 8 are sectional views of structural parts of the unit taken on lines 7-7 and 88, respectively, of Figure 6; and

Figures 9 to 15 are views showing structural details of the apparatus.

A principal use of the apparatus of the present invention is the conversion of impulses of electrical energy into sound, and in such a function the apparatus is essentiallyan electric motor energized by these impulses of electrical energy which are present in the receiving apparatus and which serve to drive or vibrate-correspondingly a suitable sound radiator or diaphragm to thus generate the sound. In common with cer I tain apparatus of this same general class which has been used heretofore, the present unit is a driving motor of the balanced armature type wherein an armature of a magnetic material is suspended between the poles of a magnet and is caused to move in that space in a manner which bears a definite relation to the incoming impulses of electrical energy which actuate it and which represent the sounds intended to be reproduced. The moving armature is connected wlth the sound radiating diaphragm and thus serves to vibrate the diaphragm in a definite man-v One important A ner to thereby generate sounds the character of which are determined by the manner in which the driving armature is actuated by the incoming signals.

The parts of the unit are supported principally from a main frame member 1, which in turn is secured to the cover 2 of the apparatus housing 3 so that when the cover 2 is removed from the housing 3 the entire working mechanism of the apparatus is withdrawn from the housing as a unit. The

cover 2 is provided with a hollow extension portion or throat 4 upon which the usual sound amplifying horn may be mounted. Covering the inner end of the throat portion 4 is the sound radiating diaphragm 5which is driven by the unit enclosed within the housing 3 and which serves to generate the sounds which are radiatedfrom the apparatus. The present unit-is adaptable to the driving of any of the well-known types of sound radiators and is not in any sense restricted'in its application to the use of the amplifying horn. In applying the unit to other types of sound radiators the structural details of the enclosing housing and the mounting of the unit are appropriately modified, but the essential structure of the operating parts of the unit remains unchanged. For example, in applying the unit to the cone type of sound radiator, the entire unit may most conveniently be housed within the cone, and the cone itself is the radiating diaphragm which is driven by the unit and which corresponds to the diaphragm 5 in the structure here shown.

The main frame portion 1 supports the permanent magnet 6 whose function is to provide the permanet magnetic field r.equisite to the operation of the apparatus. For operational and manufacturing reasons a permanent magnet is usually preferable to an electromagnet for this purpose.

The opposite arms of the magnet are provided with suitable pole pieces 7 and 8. Eachv of the pole pieces 7 andv 8 is divided to form pairs of arms 9, 10 and 11, 12, respectively, which extend into proximitywith each other as shown in Figure 6; The

their center lines substantially coincident with the center line of the air gap between the adjacent pole faces, as shown in Figures 5 to 7. The respective spring members 14 and 15 are secured at their outer ends to the frame'member 1, as indicated at 16 and 17, respectively, and theirinner ends are slotted to fit over and support the armature b 13 in the manner indicated at '18. Thus the armature 13 is supported rigidly against bodily movement in the direction the length. of the pole pieces 7 and 8 and is tiltabl'e in the air gap between the 10 pole faces of the magnet. Such tiltingmovement of the armature is opposed by a twisting flexure of the springs 14 and 15, the springs serving to normally hold the armature in its neutral position centrally 15 disposed in the air gap between the adjacent pole faces of the magnet and extendmg in a direction parallel to the center lines of the air gap so that it is equidistant from each of the four pole faces at the ends of the arms 9, 10, and 11, 12. The armature 13 is connected to the driving rod 19 which also has an attachment with the sound radiating diaphragm 5. Movement of armature 13 is therefore transmitted to the diaphragm 5, with the result that movements of the armature are converted into sound waves which emanate from the dia phragm. The actuating coils 20 which carry the electric current constituting the impulses of electrical energy supplied to the unit are mounted, as shown, in such manner that they surround the armature 13 as a core and extend through the spaces between the arms of the pole pieces 7 and 8 on opposite sides of the springs 14 and 15.

The permanent magnet 6 provides at all times a magnetic field in the gap between the pole pieces 7 and 8 in which the armature 13 is suspended. With the armature 13 j 4o balanced in the center of the air gap it is equally distant from all of the pole faces of the magnet, and the pole structure is symmetrical on the-two sides of the armature so that the magnetic field established by the permanent magnet does not tend to tilt the armature 13 out of its neutral position. In operation, the binding posts 21 are connected to the output circuit .of the receiving apparatus, as, for example, to the output circuit of the vacuum tube which constitutes the last stage of audio frequency amplification in a radio receiving set so that the electric current from the receiving apparatus, which current embodies the im ulses of electrical energy which represent tiie sounds intended to be reproduced, traverses the coils 20 which are in electrical connection with the binding posts 21. When an electric current flows in the coils 20 the armature 13 becomes magnetized to a degree depending upon the magnitude of the current. The armature 13 is in effect a core for the coils 20 and therefore, at any instant when a current flows in a given direction through the coils it is mag netized definitely with one end constituting a north pole and the other end constituting a south pole. The two coils 20'a'rewound so that the magnetic efi'ect produced by each coil is in the same direction as that produced by the other. A reversal of the direction of the current in the coils 20serves to rethe armature 13 with. opposite magnetic poles at its respective ends, as indicated by the letters N and. S on the drawing, and if the pole piece 7 of the permanent magnet is a north pole and if the pole piece 8 is a south pole the armature 13 will be tilted out of its neutral position in a counter clockwise direction as viewed in Figure 6, with a resuiting fiexure of the springs 14 and 15. The coils 26 are spaced apart a sufficient distance so that the springs 1 f and 15 are free to flex to any degree attained in the normal operation of the apparatus without contacting with the coils. The tilting of the armature takes place about an axis which passes through the armature at the points where it is supported by the springs and which is designed to occur at the geometrical center of the magnetic structure so that the entire pole piece structure is symmetrical with respect tothe axis. This tilting movement of the armature 13 may be explained by the fact that when the armature is magnetized with north and south poles located s indi-' cated on the drawing, the north po e N of the armature is repelled awa from the north pole face 9 of the pole plece 7 and is attracted toward the south pole face 11 of the pole piece 8 Similarly the south pole S of the armature 13 is repelled from the south pole face 12 of the pole piece 8 and is attracted ,toward'the north pole face .10 of the pole piece 7 Thus the magnetic forces exerted on the armature 13 at both sides of its axis-tend to pmduce a rotation or tilting of the armature in the same direction around the axis. vThe springs 14 and 15 are of such strength that, when the armature is tilted out of its neutral position, they about offset the pull of the permanent magnet which tends to increase the tilt of the freedom of movement of the armature is.

particularly important at low frequencies in order that armature vibrations of sufiiciently large amplitude may be produced. The output current of the receiving apparatus which is supplied to the coils 20 varies in magnitude and character depending upon the sounds which are being reproduced and which that current represents. As the current varies in magnitude, the armature 13 tiltsto a correspondingly varying extent.

VVhen the current changes direction the armature tilts in the opposite direction because an opposite magnetism is established in it.

The result that the armature assumes a continuous movement about its axis 21 which movement bears a definite relation to the impulses of electrical energy entering coils and which correspond to the sounds being reproduced, so that the armature serves to drive the sound radiator 5.1n such manner as to generate sounds which bear a definite relation to the original sounds as represented by the current impulses traversing the coils 20.

In the features just described and in principle of operation the present apparatus 18 similar to the balanced armature type of tion hasto do particularly with improvements in apparatus of this general class,

as, for instance, in efiiciency, quality of re-' production, and the like, which improvements are efiected chiefly. by improvements in the electrical and structural designpf the apparatus, improvements in the manufacture of the unit, and the like. Therefore, the physical embodiment of the invention essentially involves these improved features of structure and design, and it is principally in such features that the present unit differs from generally similar units hereto fore produced and it is in terms of such features that the physical embodiment of the resent invention must, for the most part, e described.

The main frame member 1 of the unit'is a single unitar member ofa bakelite composition, or simi ar moldable material, which is molded as a ,single piece at one operation with the pole pieces 7 and 8 in place so that the finished unit consists of the entire frame member 1 with the pole pieces 7 and 8 rigidly secured into and essentially constituting an integral part of the piece. The pole pieces 7 and 8: are laminated as shown according to the usual practice of constructing magnetic part of this nature, and the laminations are held together by rivets 22 which project at the sides of the pole pieces as shown and in the molding process become imbedded in the material of the molded frame member 1 so that they function as retaining means for holding the pole pieces rigidly in fixed relation to each other. To

, furth r assist in securing the pole pieces to nection between the parts. In order that properly operating units be produced in the course of ordinary manufacture it isnecessary that the air gap between the pole faces be accurately obtained and maintained uni- .form throughout a manufacturing run. When the molded units are withdrawn from the mold they are still hot, and upon cooling material of the member 1 shrinks.

Thus when the unit is first withdrawn from the mold the air gap between the pole faces, is somewhat greater than it will be after the article has cooled and the resulting shrinkage takes place. The proper air gap is secured by spacing the pole pieces in the mold slightly further apart than they should be in the finished article and providing a gage of thickness equal to the desired width of the air gap which gage is introduced between the pole pieces 7 and 8 when the hot unitis withdrawn from the mold. Cooling of the material causes the pole pieces to shrink down upon the gage and thus, when the article is thoroughlyi, cooled, the pole pieces have been positively set in exactly the position which produces a proper air gap. However, reasonably good results in the spacing of the ole faces 7 and 8 may be obtained without t e use of the gage for. the reason that in a standardized process of manufacture the shrinkage is always substantially the same and a proper allowance for this shrinkage may be determined upon and provided for when the pole pieces are located in the mold. With this process of manufacture wherein the pole pieces are molded directly into thematerial of the member 1 and particularly where the pole pieces are shrunk onto a gage to positively determine the width of the air gap, the spacing of the pole pieces is always sure to be correct as a matter of course without necessity of skilled labor and without resorting to a timeconsuming process of adjustmentfi This is in contrast to the usual type of structure wherein if any attempt at all is made to obtain a proper air gap and a proper relation of poles, elaborate adjustments are provided to effect relative movement of the pole pieces in all dimensions. Such adjustments while necessary in the structures heretofore employed, add materially to the complicity and cost of the unit, give rise to the danger that the unit will get out of order during use through shifting of sthe adjustments and introduce an expensive element of labor cost in putting each .unit into proper adjustment before it is in condition to be distributed to 'the trade. I

The respective arms of the permanent magnet. 6 lie adjacent the respective pole pieces 7 and 8, as indicated at 23 and 24. A close and regular contact between the main body of the magnet and the laminated pole ieces 7 and 8 is required in order to estabhsh a good magnetigcircuit. The faces nee-7,531

of contact 23 and 24 between the arms of the magnet 6 and the pole pieces are at right angles to each other. In assembling the-unit the magnet is slidinto the position shown in contact with the pole pieces and is guided and supported by the recesses in the molded member 1 into which it fits in the manner shown in the drawings. The faces 23 and 24 of the magnet which are to contact with the pole pieces are ground toa fit, the only requirement being that the faces be at right angles with each other. This is in contrast to a structure where, for instance, the parallel end facesof the arms of the permanent magnet come into contact with the pole faces, in-which case it is not only necessary that the contacting faces be ground to the proper angle but also that the arms of the magnet be of exactly the proper length. The structure of the present apparatus employing the non-parallel magnet faces sim plifies the construction and reduces the manufacturingcost for thereason that it minimizes thejnumber of manufacturing requirements which must be satisfied in order to produce a unit of good quality.

The springs 14 and 15 which support the armature 13 in the air gap between tbs pole pieces 7 and 8 are secured to and supported from molded frame member 1 by clamp's or connections indicated at 16 and 17'. The clamp 16 rests against a fiat portion of the member 1, as shown, so that it holds the spring 14 in fixed position. Clamp 17, however, rests upon' a'projecting portion 25 of the member 1 so that by rocking the clamp across the top of the portion 25 the spring 15 may be turned to thus change the angle of tilt of the armature 13 in the air gap. This furnishes a convenient and' speedy adjustment for coating the armature in its proper central position within the air gap. The proper location of the central axis of the armature is accomplished by the use of spacers, as in the ordinary manufacture of apparatus of this .general class, and by the provision in the springs of-screw holes somewhat larger than the retaining screws so that the springs and armature can be shifted a small amount. Lock washers are provided on the screws so that their position of adjustment is maintained. The adjustment for regulating the tilt of the armature is convenientl efiected by means of-thetwo screws 26 w ich may be independently adjusted to'brin the armature into its desired positionl'fhis is in contrast to the usual adjustments for the armature, frequently involving a bodily shifti g of the armature, which are provided on ap aratus' of this type. Such adjustments complicate the apparatus and require a considerable amount of time and care in assembly. Furthermore; in practice it has been found possible to provide for but a small range of adjustment. Frequently this range'is insufiicient to effect a proper adjustment, and inordinary manufacturing practice the worker often deliberately stresses the supporting springs beyond their elastic limit by twisting them to thus permanen tly deform them into such position that the armature will lie properly in the air gap.- Such treatment, of course; destroys the characteristics of the springs. The present invention provides practically an unlimited range of adjustment for the armature without necessity of flexing or bending the springs to any damaging extent.

The springs 14 and 15 are preferably tapered as shown so thattheir outer ends are wider than their inner ends which en gage the armature. This shaping makes for stiffness and affords a high degree of rigidity which prevents movement of the armature bodily .toward either one of the pole pieces 7 or 8. At their inner ends' springs 14 and 15 are slotted so that they fit over the armature 13 for a considerable distance, as indicated at 18. The armature 13 is flat and wide so that a large region 0 contact between the slotted springs and the armature is provided. This contact is sufficiently extensive so that a sup orting connection of adequate strength between the springs ,and the armature ma be formed by the use of solder of low me ting temperature. The heat of soldering is so limited that its does not damage the specially hardened metal of the sprin s or impair the magnetic properties of t e armature, and thereforethe character and properties of the springs and armatures are the same after the apparatus is completely assembled as they were at the time'the parts were made. Thus the springs can be made up originally of exactly the proper strengt an stifi'ness, and they may be so designed that awide range of tilt of the armature is possible without stressing the metal of the springs beyond its elastic limit. The possibility of making up initially springs of exactly the proper-qualities, the fact that the spring are not injured durin ufacture .of the unit and the act that a wide range of tilt of the armature is'possible with such springs without injuring them all make for a proper response and control of the armature and a correspondingly increased excellence of reproduction. l-Ieretofore, where this type of spring support was used a relatively narrow armature was ordinarily employed, usually being fairly thick. The narrow width of the armature restricts the amount of contact which can be efi'ected with the supporting springs, with the result that in order to effect a connection of suflicient strength it is necessary to em loy a high melting temperature solder. uring the soldering proc' ess the high temperat res attained soften theinetalof the springs. With the metal .thus softened a lesser stress is required to deform the metal beyond. its elastic limit, and such stress will permanently deform the metal. Hence, if the springs used in the manufacture of the-apparatus are initially of the proper strength they become so softened and weakened by the heat of the soldering process that stresses which the original springs would have satisfactorily withstood and which are normally attained in the operation" of the apparatus are of sufficient magnitude to stress the springs beyond their elastic limit, thus permanently deforming them and rendering the operation of the apparatus generally unsatisfactory. Accordingly, the general .practice isto use springs which are originally too stifi so that after the soldering process they will still be of suflicient strength to be serviceable. The soldering operation on different units affects the springs to different degrees and hence no particular uniformity in the product can 'dimension-ofthe armature which is perpendicular to the plane of the coils 20 and by the width of the armature is meant the other principal dimension of the armature which extends at right angles to its length. Being wide the armature can be thin and still present a substantial cross section area. It is important that the armature be of considerable width and of relatively little thickness, whereby it more. nearly approximates in appearance a flat sheet than a-block or body of any.considerable thickness. "This shaping of the armature is particularly important in the matter of restricting power loss in the apparatus and of preventing distortion of the reproduced sounds. 'The changing magnetic flux produced in the iron core of a coil which is carrying a varying current, as are coils 20, induces electric currents in the metal of the core. These induced currents or eddy currents, flow in closed paths which encircle the axis of the coiland which are generated in planes that are parallel to the plane of the coilfthat is, perpendicular to the direction ih whichthe magnetic flux passes through thecore. Looking at a section of the armature taken parallel to the plane of the coils, as in Figure 5, the armature appearsas an elon'gated/strip, and it maybeseen that the average closed path encircling I the central axis of the armature and coils is of much greater length than would be the case if the armature were more nearly square 1n cross section. Thislong path introduces a correspondingly high electrical resistance opposin the ow of the induced eddy currents an thus'reduces in direct [proportion the amount of these? undesirable This feature of the present inventortion in the ultimate sounds produced by it has been proposed to laminate the armature, but such an expedient is not successful due to the fact that it adds to the manuthe fact that a composite armature structure will not satisfactorily yibrate at high frequencies as an integral unit. The short length of the armatfureialso correspondingly reduces the loss and distortion arising out of undesirable currents induced the armature. These difficulties incident upon the induced currents are directly proportional to the length of the armature for the reason.

that the currents flow in planes which are perpendicular to the length of the armature and the width of the current path is therefore the length of the armature. By shortening the armature and thus reducing the width of the current path the conducting the apparatus. To overcome this difficulty facturing cost of the apparatus and'due to.

property of the armature with respect to' .the'se currents is reduced in the same proportion. Furthermore, the minimizing of the length of the armature limits'itsmass and'br'ings nearer to its axis of rotation the average mass of each of the two halves of the armature which lie on opposite sides of the axis with the result that the armature, particularly at the higher frequencies, vibrates more nearly in exact response to the Va ing current traversing thecoils 20.

he present structure employs a relation between the'transverse cross sectional area ,of the armature and the area of the pole faces which is radically different from that heretofore employed in apparatus of this general class. For the best results, a high magnetic flux density should be maintained in the metal of the pole faces for the reason that with such a high flux density a iven current" in the actuating coils 20, through the 1 medium of the resulting magnetic' flux generated in the armature; produces a greater movement of the armature and of the diaphragm'awith the production ofa correthe pole pieces just reach point when they are carrying the maximumamount of magnetic flux whlch the'apflaratus demands. The arms 9, 10, and 11, 12 which constitute the pole faces are tapered, as indicated at 28, to concentrate the permanent magnetic flux in the air gap and to aid in effecting a uniform distribiition of magnetic flux throughout the pole faces.

The actual size of the pole faces depends magnetic flux density in the armature invariably reaches the saturation value at relatively slight positions of tilt of the armature out its normal neutral position. -As the armature tilts its respective ends approach more closely to the respective pole pieces 7 and 8 with the result that an increased amount of magnetic flux traverses the armature. Saturation of the armature at degrees of tilt frequently attained in the normal operation of the apparatus introduces harmonics into the sound output of the apparatus and consequently produces a distortion of the reproduced sound. Furthermore, when this condition-of saturation in the material of. the armature exists, a given power input to the apparatus produces a lesser tilting of the. armature with the result that a larger power input for a given sound output isrequired. As a corollary'to this proposition a larger power input is required to effect a given tilt and consequently a given sound output, with the result that in order to ob-,

tain a satisfactory sound output the ampli: fying tubes are often required to handle a larger amount of power than can be done satisfactorily so that over-loading of the tubes and distortion of the reproduced sound results. This is particularly the case for the lower frequencies In the present invention the cross section of the armature is prefer-v ably substantially equal to the area. of each of the pole faces provided on the arms 9, 10,

11 and 12. Fairly good results may be obtained with an armature cross section of somewhat less than this amount, but for the best results the ratio of pole face area to armature cross section should be approxi mately unity. In the structure here shown the armature may tilt practically as far as the mechanism of the apparatus allows without passing the saturation point. The result is that an improved quality of repro duction is effected and a large'sound. volume output is obtained for a given power input.

The sound radiating" diaphragm may assume any appropriate form, as, for in-- the saturation stance, a flat diaphragm, a cone or the like. It is here shownas a cone ribbed along its surface as indicated at 29 and provided at the open endof the cone with an integral flat disc portion 30 having formed in it circumferential corrugations 31. The ribs 29 add rigidity to the cone portion of the diaphragm while the corrugations 31 in the disc portion 30, which is secured as indicated at 32 to the cover plate 2, afford a flexibility of the unit-which permitsthe rigid cone portion to vibrate bodily as a rigid unit, all of the flexure of the diaphragm 5 occurring in the flat disc portion 30.

A single driving rod 19 connects the armature 13 directly with the sound radiating diaphragm 5. This direct drive is possible due to the fact'that, as above described, the armature 13 has awide range of tilting movement in the air gap ahd thus vibrations of sufficientlylarge magnitude are imparted to the diaphragm 5 by the direct drive. This avoids the mechanical. linkage or extended portion on the armature frequently employed in the driving connection between the armature and the diaphragm because of the fact that the amplitude of movement of the armature is so small that its motion must be magnified in order that it will impart a sufficient movement to the 'diaphragm.-

Such connect-ions introduce frictional loss ortransition loss due to improper loading of the driving unit and add to the mass of the moving parts with the result that, particularly with high frequencies, the reproduction becomes inaccurate. I have found that important advantages are obtainable by employing a hollow thin walled tube as a driving element. This is in contrast to the usual solid metal rod. This hollow tube driving element affords a greater stiffness and resist ance to bending than does a solidrod embodying the same weight of material. The driving element 19 of the present structure is of less weightthan the usual solid rodemployed in units of this general class and at tion of .the driving linkage, and the like.

With the usual solid driving rod much diffi culty is encountered in breaking of the rod or the solder joint at the point where the rod is connected to the armature. As the armature tilts and actuates the dia phragm through the medium of the rod a certam flexure of the rod isproduced, and in the ordinary structure this bending action .tends'to'break the rod near its end adjacent the armature"or to break open the solder joint between the rod and the ar'inature. In-

I the present'apparatus the upper end of the rod 19 is flattened for a short distance as indicated at 33. This serves to provide a large area of contact between the rod and the armature and thus permits a very substantial solder joint. Also, the flattened upper end of the rod is in effect a thin strip of material and readily takes up the bending action which the rod' must absorb during operation of the apparatus without concentrating the bending stress on the solder joint 'or on any particular point in the rod to such extent that breakage'will occur. The flattened portion of the rod extends throughout onlya small portion of the length of the rod and is furthermore removed from the middle of the length of the rod so that no objectionable sidewise vibration or lack of stiffness is introduced because of this feature. The end of the driving rod 19 which is adjacentthe diaphragm 5 is passed through a hole at the apex of the cone portion of the diaphragm and its end portion which extends within the cone is flared out so that the metal of the tube lays against the inner face of the diaphragm over a substantial area as indicated. With this large area of contact between the end of the driving tube 19 and the diaphragm 5 a substantial joint is effected by merely dropping a small quantity of an appropriate wax over the flared end of the tube, as indicated at 40.

" This avoids the inconvenient and complicated threaded nut and washer connection which is ordinarily employed with the usualsolid rod driving element, and thus further reduces the mass of the moving parts.

-The apparatus of the present invention is also operable as a generator of electrical energy. For example, in phonographic reproduction from the usual permanent grooved record the present apparatus may be used as a reproducer. The armature of the apparatus is placed in hysical connection with the stylus which follows the grooves in the record, and the motion derived by the stylus in assing over the impressions on the record armature: The resulting movement of the armature in the permanent magnetic field of the apparatus generates a changing magnetic flux in the armature, and such chang ing flux generates a correspondingly varying current in the electric current conducting coils surroundin the armature so that I the current in the 001 s thus bears a definite relation "to the impressions on the phono-.

graph record over which the stylus has passed. The present apparatus is'particularlyjwell adapted to this use for the reason that its structure andoperating characteristics, such as thekindof armature employed, and the like, make possible the attachment of the armature directly to the stylus withis impa'rted to the out the interposition of a mechanical linka or its equivalent as is necessary in this 1 general class of apparatus heretofore used.

The varying current generated in the current conducted winding may be amplified by vacuum tubes in the usualmanner and the amplified current then converted into sound by passing it into a second unit which-acts turewith the result that a current is gener-.

ated in the coils, which ,currentmay be conducted over wires to any desired point and then reconverted into sound by passing it into a second unit in the manner described. The present invention therefore provides a sound reproducing apparatus, and more particularly a unit for converting energy furnished to the unit into a different form, which, in operation exhibits certain advantages over existing units of the same general class, notably an improved quality of reproduction and an improved efliciency, as, for instance, in the matter of volume of sound output derived from a given power .input. For the most part these advantages are achieved by improvements in design, both as to the mechanical structure and as to theelectrical properties .of the unit. The

structure permits of easy and inexpensive. -manufacture and afi'ords a compact rugged high power unit. The whole structure permits of manufacture to close limits as regards the ultimate sound reproducing qualities of the device, without, however, requirin other than simple and rapid manufacturing and assembling operations. The

manner. in which the main frame memberof the unit is molded with the pole pieces in place insures by a quick and easy method of manufacture a proper air gap in each unit of a manufacturing run. The construction and manner of fitting the permanent magnet to the pole insures a, proper magnetic circuit at a very low manufacturing cost. The structure of the armature and .its connection to its supporting'springs insures a proper and extensive spring .action throughout theentire wide range of:

movement of the armature, while the ad justment for locating the position "of the armature affords a quickand ready means .for getting the unit into. proper ad ustment} without injuring any 0 the parts. The

magnetic structure-of the armature restricts power losses and distortion. The magnetic structure of the pol-epieces and the prov fsion of a proper relationbetween the mag-i netic properties of the armature amid the pole pieces efi'ects an improved efiiciency and an improved quality of reproduction together with the other advantages that these featuresimply, as,-for instance, the ability to obtain a satlsfactory sound volume without recourse to power supply sets aiid'power tubes, and the like The wide amplitude of movement of the armature permits of a direct drive of the sound radiating diaphragm, and the type of driving connection employed offers improvements in quality of reproduction, ease of manufacture and durability and ruggedness of the unit; The invention contemplates these various improvements of structure and design and the claims are, for the most part, necessarily worded in terms of these features. The invention broadly contemplates the apparatus in so far as it may be characterized by its improved manufacture and operation and by the structural features which make possible these ferring relation with said movable armature, a sound radiating diaphragm, a hollow rod connectingsaid armature with the diaphragmto thereby vibrate .the said dia- V phragm in definite relation to the movement of the armature, a flared portion at that end of the'hollo'w rod which connects with said diaphragm for presenting an extended area of thefmaterial of the rod in contact with the diaphragm, and a connection between the said rod and. the diaphragm which consists of-a solidified mass of a low melting temper- Y ature cementing material covering the flared "portion of the rod and-the adjacent'material of the diaphragm.

no r

2. Soundre'producing apparatus of the class described which comprises means for producing a permanent magnetic field, an. p

armature mounted for tilting movement in said magnetic field andtadapted in regular operation of the apparatus 'to assume a vibrational tilting movement at sound frequencies, an energizing electric current conlation with sai ducting windingin energy transferring reelectric current which by induction energizes said armature and effects a movement thereof I which bears a definite relation to' the character of the current-traversing the said winding, a sound radiating di-aphragm, a

thin walled hollow" rod connecting the arma ture with. said diaphragm to thereby vibrate the; diaphra in definite relation to the movement o armature for carrying an.

'of elongated ingcapacity comparable to that of each of the armature, a flared portion at the end of the said hollow rod which connects with the diaphragm, a connection between the said end' of the rod and the dia- V ture, whereby the bending action of the rod is absorbed insaid flattened portion and the stiffness of the rod to lateral vibration is not disadvantageously' affected.

' 3. Sound reproducing apparatus of the class described which comprlses a permanent magnet, pole pieces for said magnet forming between them an air gap, an armature of magnetic material mounted for a limited rotational movement through said air gap in magneticflux transferring relation with said pole pieces, pole faces provided on said pole pieces at. the said air gaps of sufficient flux conducting capacity to carry without saturation: the maximum quantity of magnetic flux normally required for a satisfactory'energy output, the said pole faces and thesaid armature being of comparable flux and a slopedportion onv carrying capacity, said polepieces constituting a tapered. end for the pole pieces terminating at the said pole face and extending back in one direction from said pole face at a relatively sharp angle to the direction of extent of said pole face to form the full section of the pole pie ce of at least twice'the area of the pole face.

4. Sound reproducing apparatus of the class described which comprises a permanent magnet and pole pieces having pole faces of elongated shape and of magnetic flux conducting areasomewhat lessthan the flux conducting area of, the said pole pieces back of the pole faces, whereby a flux density in the said pole I faces substantially equal to thesaturation value for the said pole faces can he used without reaching the saturation point in the said pole pieces .back of the ole faces, but of substantial area to'carry, without objectionablypassing the saturation point, the maximum effective quantity of magnetic flux which will normally pass from the pole pieces during operation of the'apparatus in generating mder the conditions normally required of apparatus of, its class, that quantity of sound output which is the most advantageous for the listener,

and a tilting armature operating iii-magnetic' flux conducting relationv with said pole pieces of cross sectional area also shape and of" flux carrysaid pole faces so that for all positions of 'tilt normally attained by said armature, due

ing into proxinlity with corresponding arm portions of the other pole pieces, thus form-- ing air gaps between the adjacent ends of M the respective arm portions, pole faces provided at the saideadjacentends of said arm portions extending substantially at right angles to the normal direction of passage of magneticfiux across the air gap between the two pole pieces, a sloped portion provided on the outer face of-each of said arms extending back from the pole face so that the flux transmitting area of each of the pole faces is less than'the flux carrying cross sectional area of the body portion. of the arm back of the pole face, and an armature consisting of a single piece of magnetic ma- I terial tiltably mounted in the air gaps between the pole pieces in flux conducting relation with said pole faces and having a sizeable cross sectionalarea sufficiently large to carry, without reaching the saturation point, such quantity of magnetic flux as is necessary to produce a sound volume output commensurate with all reasonable requirements of apparatus of this class, having roughly a one to one ratio of cross sectional area of each ofsaid pole faces, and presenting such cross sectional areaas an elongated rectangle so that the armature is in-the form of a wide thin stripof width at least several times its thickness," V

6. Sound reproducing apparatus of the class described which comprises a permanent magnet with pole pieces forniingan air gap between them and sloped toward their ends so that the flux carrying area of each of the pole faces at the air gap is substantial and adequate for a'respectabl'y large sound outputwithoutmagnetic saturation at the pole faces but is less than the flux carrying cross sectional area. of the pole piecesback of the said pole faces, and a short, wide, reasonably thin armature of magnetic material mounted for a partial rotative movenient in the air gap between the pole pieces of the magnet so that its position with reference to the respective pole faces is .diflerent at different positions in its range of movement, and'havor er of magnitude as that of the said pole faces so that When the flux in the pole faces is approximately at saturation value the flux in the armature will not susbtantially exceed the saturation point for all positions of tilt attained during the production of a reasonable required sound output. 1

7. Sound reproducing apparatus of the class described comprising apermanent magnet having pole pieces with two pole face carrying portions forming air gaps between them, and an armature tiltably movable in said air gaps consisting of a short, Wide reasonably thin piece of magnetic material havinga substantial total transverse cross sectional area extending at its ends beyond the'inner sides of said pole face carr ing portions but terminating short of tie outer sides of said portions so that the principal pole face area effective in transmitting magnetic flux between the respective piece portions and the armature is less t an the flux conducting across sectional area of the said portions at a point back of the olefaces but is a substantial area so that w en the apparatus is operated with the high flux density in the air gaps between the pole faces and the armature which; is required for good efficiency and is operated to deliver the-proper sound output commonly required of apparatus of this class to produce the best effect on the listener the said ole faces and the armature are not subjec to sub-.

stantial magnetic saturation.

8. Sound reproducing apparatus of the classdescribed which comprises a permanent magnet having pole pieces arranged so that, an air gap is formed between them, pole faces at the said air gaps of sufiiciently great' magnetic flux conducting capacity as to produce a. satisfactory energy output without objectionablesaturation, aflat single piece armature mounted for a limited tilting movement in the air gaand having a cross sectionalarea sufficient y large so that with the armature inthe "maximum position of tilt normally required for, the generation of a sound volume output sufficiently great to satisfy all reasonable demands upon theapparatus and with approximately the maximum effective flux between the said pole faces and the armature that can practicably be attained without passing the magnetic saturation point at any section vof the said pole pieces, the flux density inthe' armature ole does not substantially pass thesaturation point. BEN BROWN. 

